It sounds enticing, what with being able to run their famous hardware plug-ins, if I understand that right...But as the 'one man band' type who has gazillions of ideas he wants to turn into songs, I'm wondering if the $699 price tag is overkill for what you get, versus say the latest generation Focusrite 2,4,6 or 8 input Scarlett line....like  would it be good enough with some decent soft plug ins. Obviously the much lower price point is a factor too.

I would say if you can afford it, go for it.
 

interpolated
I would say if you can afford it, go for it.
 

Well, it's not just the unit he'll need to afford but all of the UA plugins he'll want afterwards. 
 
I've purposely avoided UAD hardware because I know that it would cost me a LOT MORE down the road. 
 
Same with Slate plugins.  I recently sold all mine after realizing I rarely used them but still lusted after every new plugin they released.  They're great plugins, but the madness for me had to stop somewhere.
 
 
 

The Apollo is a great, professional unit.  UAD effects are great.  But as noted above, UA sells their plugs hard and are always coming up with new (and more processor intensive!) plugs.  It is like they design them to force you to buy a better unit w/ more shark chips ;-)
 
With that as a warning, I don't see any other problem.

The ONLY thing I regret about buying the Apollo Twin USB is that I didn't wait and buy an 8 channel Apollo.

Disclaimer - I've been using UAD plugins since the days when the UAD card was distributed by Mackie. There has to be some bias<G>!
 
That said, for emulations of hardware I think UA is the best. Can you hear the difference between their 1176 and the 1176-ish compressor in Sonar? I think so, but it is subtle, and you may not need the UA version. Or you may. The thing that I like most about the UA emulations is that they react the way the hardware reacts. Let's face it, no one really KNOWS what an 1176 sounds like, even when they were first made there were variations - component tolerance, age, design... but we all know how we expect an 1176 to react to certain inputs. If that makes sense...

The guitar amplifier emulations that support the Unison front end in the current Apollo interfaces is really reamarkable Again, no two Fender 5E3s or Marshall JCMs sound alike, but we have a certain expectation about how they sound and how they react to our playing, and the UA Unison plugins are really close (no, I'm not selling my amplifiers or microphones yet - but I do enjoy playing through the plugins!)

And then there are my secret weapons - the EMT 140 plate is probably better than any physical plate I've ever used. Certainly more flexible<G>! And the Ocean Way Room plugin will blow your mind. I still grin every time I use it.

So yes, I'm an enthusiastic UA customer, maybe even a fanboy?

The Apollo series have really good microphone preamplifiers, even better converters, and the plugins are part of my workflow. I will add an 8 or maybe even 16 channel Apollo when I can.

And yes, that initial purchase is just that, you will want more plugins. They have a sale every month (EQs, Top 10, etc), and a huge sale at the end of the year - or at least that's what they've done so far. I own quite a few of their plugins and by watching for sales, and using vouchers (which they provide from time to time) I've been able to expand the collection without selling any kids!

Absolutely worth it!

I have an aging dsp card with only some of the uad plug-ins. Can't fault them. Ideally I want to go with an external solution now so I can get the best of both worlds.

Thanks all, that gives me some stuff to chew on

I am a huge UAD fan - an Apollo Silverface Quad, 2 PCIe Octo cards, and a ton of plugins. I'm even chewing on adding an Apollo 16 to the mix. Using the Neve 1073 in Unison mode for vocals is just killer.
 
I used to have some problems with the drivers but UAD have really improved the install, the drivers and their console program.
 
The only real downsides are cost, and the fact that you can max out your processors fairly easily. Other than that, they have FANTASTIC plugins. 

Like the Madonna cover on your Soundcloud page.